1. Field of the Invention
This invention relates to an electrosensitive medium which can form a visible image directly from an electric signal.
2. Discussion of the Prior Art
A variety of recording media are known which form a visible image directly from an electric signal but they have various drawbacks.
For example, in one known recording method, recording is effected by subjecting a conductive solution or an electrolyte impregnated in a paper sheet to electrolysis in response to an electric signal. In this method various difficulties have been experienced including (a) great variations in recording characteristics due to the water content of the paper sheet, (b) wear of the electrodes, (c) fading of picture images, and (d) discoloration of the recording paper sheet and the like.
In order to eliminate these difficulties, a variety of dry type electrolytic recording media, as disclosed in Japanese Patent Publication No. 38-22341, have been developed. However, none of this prior art recording media is satisfactory in that they invariably contain a large amount of a metal oxide or a metal sulfide in the coloring layer of the recording sheet so that wear of the recording stylus is accelerated due to the substantial hardness of the coloring layer. Further, a metal oxide or a metal sulfide which permits easy color development will have its own color and therefore is not suitable for coating on a recording sheet since the latter has imparted thereto the particular color of the metal oxide or sulfide used. Also, a recording sheet using a white metal oxide has drawbacks in that it is susceptible to non-uniform recording and lowered image density because of its increased electrical resistance.
There has also been used in another embodiment of the prior art an electrosensitive medium comprising (a) an underlayer of carbon or some other dark colored conductive material coated with a binder on (b) a substrate or a supporting means, (c) a white surface layer being disposed on the dark colored layer. With such a recording medium, portions of the white surface layer are burnt and removed by sparking to form a picture image by means of the contrast between the non-sparked white surface layer and the dark colored underlayer. However the burning operation produces a bad odor, smoke and ashes to the potentially extreme discomfort of the operator. Moreover, the white surface layer must have a thickness as large as 10 to 20 microns in order to conceal the color of dark colored underlayer with the result that the recording medium suffers from lowered recording sensitivity in addition to the difficulties mentioned above. Hence, application of high voltage is necessitated which tends to disintegrate the dark colored underlayer into powder thereby causing recording failure because of deposition of the powder on the recording stylus. Further, the powder deposited on the recording stylus is transferred again onto the white surface layer in the form of a so-called "tail". Also, a portion of the powder is directly dropped onto the white surface layer without depositing on the recording stylus and spoils marginal portions of the picture image.
An example of prior art which overcomes these difficulties is found in Japanese Patent Publication No. 44-31998 where there is disclosed a recording member comprising a base layer such as a paper sheet, a colored layer disposed on the base, a thin metallic layer disposed on the colored layer and a white surface layer which can be very thin. However, the recording medium disclosed in this patent publication is disadvantageous in that it causes accelerated abrasive wear to the recording stylus. Thus, it cannot be practically used for stylus-scanning type recording due to tone variations. Therefore, the recording medium of the above Japanese patent publication has been exclusively used for the dot matrix type recording.